Reference voltages are widely used in electronic circuits especially in analog circuits where electrical signals have to be compared to a standard signal, stable with environmental conditions. The most adverse environmental factor for circuits on a chip is temperature. A reference voltage based on the bandgap principle consists of the summation of two voltages having opposite variations with temperature. The first voltage corresponds to a forward biased p-n junction having a Complimentary to Absolute Temperature (CTAT) variation with a drop of about 2.2 mV/° C. The PTAT voltage is generated by amplifying the base-emitter voltage difference of two bipolar transistors operating at different collector current density. A first order temperature insensitive voltage is generated by adding a CTAT voltage to a Proportional to Absolute Temperature (PTAT) voltage such that the two slopes compensate each other. If the PTAT and CTAT are well balanced, all that remains is a second order curvature effect, which may be compensated for as required by inclusion of additional circuitry.
While such circuits offer temperature insensitive reference voltages they suffer somewhat in that they are affected by voltage noise on the resultant reference voltage. As it is known to those skilled in the art, the voltage noise on a reference voltage has two components. A first component called low band noise, or 1/f noise or sometimes referred to as flicker noise typically has a contribution in the range from 0.1 Hz to 10 Hz. A second component referred to as high band noise, or white noise typically has a contribution over 10 Hz.
A major source of the low band noise in bandgap voltage references based on bipolar transistors, which is not easy to compensate, is generated by the bipolar base current and in order to reduce this noise the base current has to be reduced. One solution to reduce the base current and the associated 1/f noise is to use bipolar transistors with very high gain, which is the ratio of collector current to base current, usually called “beta” factor. From a cost or efficiency point of view it is always preferable to design a circuit using normal processes where “beta” factor is typical of the order of one hundred. Such beta factors are not typically sufficient to compensate for the low band noise.
The high band noise is generated by collector current such that the higher the collector current, the lower the high band noise. In order to reduce high band noise collector (and base) current have to be increased. As a result the operation conditions required to minimize low band noise and high band noise are opposite to one another. This makes it difficult to achieve circuitry which can minimize both these noise contributions simultaneously.
There are therefore a number of problems associated with generating voltage references with low noise contributions.